The project is designed to answer clinical problems in the management of patients, particularly those with the acute respiratory distress syndrome (ARDS). The objectives are, first, to discover whether the pressure in a segment wedged by a balloon catheter acts in a way similar to an unoccluded segment which is influenced by the effects of the alveolar microvascular waterfall and the pulmonary venous waterfall. This is to be studied in the open and closed chested dog with pulmonary edemas of the hemodynamic and "capillary leak" types. The effect of the advent of this edema on the compliances and resistances of the arterial, microvascular and venous segments will be studied using very thin retrograde catheters which measure side pressures of the small vessels within the lung. The second objective is to study the relative significance of two factors--the fall in venous return and the increased vascular resistance of the lung--in causing the fall in cardiac output which occurs with positive alveolar (and expiratory) pressures (PEEP). This should be clarified by measuring cardiac output while varying intrathoracic pressure only. The lung volume will be kept constant by bilateral pneumothoraces pressurized by the same amount as the increase in alveolar pressure. Using the same model, these cardiac output changes caused by intrathoracic pressures will be contrasted with those associated with volume alterations at the same intrathoracic pressure. The significance of these studies is that if the factors affecting wedge pressures during mechanical ventilation are understood, it may be possible to check the veracity of such measurements when used in the management of patients with ARDS who are being treated with PEEP. It is also clinically important to know the effect on the cardiac output of such patients of volume changes, which depend on compliance and recoil, versus alveolar pressure changes which only depend on the ventilator pressures (PEEP).